Sensor units of this type constitute so-called air quality dual sensors, which can carry out a carbon monoxide and a nitrogen oxide measurement for quantifying the exhaust gas quality of motor vehicle internal combustion engines. In modern internal combustion engines, a multiplicity of input parameters is processed in the course of electronic engine management. These are, for example, the instantaneous engine revolutions per unit time, the gas pedal position, etc. A further characteristic quantity for engine management is the pollutant concentration of the exhaust gases arising from combustion. These pollutants, as is conventional, are measured using sensors, the data from the sensors being taken into account in the engine management.
International Patent Application WO 93/10441, describes a conventional method which, for determining small quantities of carbon monoxide and nitrogen oxide in oxygen-containing gas mixtures, makes use of a sensor, whose electrical resistance changes in response to an increase in temperature. The magnitude of the change is a function of the carbon monoxide and nitrogen oxide concentration in the gas mixture. Therefore, the change represents a measure for these pollutants. For each pollutant, a separate sensor is used, an electrical heating element being assigned to each of which for traversing the temperature range.